Aux fan thread..oh boy

[lolcantturn]

I'm the owner of an M42 car now, being as both of my previous E30's were both M50s I would never have this problem as I just put an electric fan and never dealt with an Aux fan. This car is very original and OEM so instead of just adding an electric fan that would make my life 10x easier, I'm going to keep the Aux fan. That being said, my aux fan doesn't turn on with AC or with just the temperature rising so I kinda run hot in traffic which worries me.

The resistor appears to be in good condition, looks fairly new actually. With the ignition in just ON, engine not running, AC button NOT on, I can get the fan to turn on jumpering the green/black wire to the brown/black wire, that's on high mode. I can get it to turn on on high mode as well with the AC button on, but when I have the AC button ON and I try jumpering the fan to the low setting, it doesn't move. This is with the engine OFF.

So I'm just going to conclude that the fan switch is the culprit? It looks like it's had better days. Just wondering as I want to run to the stealership tomorrow to pick one up.

[jlevie]

If the engine runs hot when A/C isn't being used, there is another problem that needs to be fixed. The causes of high temperature when idling or in stop and go traffic include a bad thermostat, bad fan clutch, or partially clogged radiator. Start by replacing the thermostat and fan clutch. If that doesn't eliminate the problem, replace the radiator. Use only new parts.

You've established that the aux fan works (it will run on high speed). But it didn't run when you jumpered the low speed lead of the thermo switch. That says that the resistor is open, the low speed fan relay is bad, or there is a wiring fault in the low speed circuit. You can swap the low and high speed relays to see if that is the problem. Disconnect one side of the resistor and use an ohmmeter to see if the resistor is good.